The present invention relates to a gas-venting arrangement incorporated with a mold for use in a molding machine, such as a die casting machine or an injection molding machine, particularly to an improvement of the gas-venting arrangment of the type disclosed in Australian Pat. No. 516,938 and U.S. Pat. No. 4,431,047 to Takeshima et al., which issued Feb. 14, 1984.
The gas-venting arrangment incorporated with a mold, disclosed in U.S. Pat. No. 4,431,047, is illustrated in FIGS. 1 and 2 of this application. Referring to these figures, reference numerals 1 and 2 represent stationary and movable platens, respectively. A mold is shown consisting a stationary mold half 3 and a movable mold half 4. A cavity 7 to be filled with a melt is defined by the mold halves 3 and 4. The mold is provided with a push plate or ejector 5 and push pins 6. A molten metal casting hole 8 is formed in the mold to communicate with the cavity 7. A shallow but wide groove is formed in the movable mold half 4 in an area on the periphery of the cavity 7. The shallow groove and a flat parting face of the stationary mold half 3 facing the groove define a thin gas vent passage 9 in the mold. An additional gas vent passage 10 connected to the top end of the thin gas vent passage 9 and extending upwardly or rearwardly is formed in the mold. The additional gas vent passage 10 lies on the parting faces of the two mold halves 3 and 4. In other words, it has a cross-section taken along a line parallel to the axis of the mold, the shape of the cross-section being defined by the two mold halves 3 and 4. At the opposite end of the gas vent passage 10 from gas vent passage 9 are a valve chamber 11 that can be split into two parts, a valve seat 12 and a gas discharge passage 13 having an outlet 20 opening to the outside of the mold which are formed in the mold, so that they are arranged upwardly in series on the parting faces of the two mold halves 3 and 4. A slide valve 14 capable of sliding in the vertical direction is disposed in the valve chamber 11. The valve 14 has a disc-shape, and the periphery of the upper end of the valve 14 is tapered.
Two symmetrical by-pass passages 15 detouring around the valve 14 are formed to extend from the gas vent passage 10 a portion of the valve chamber 11 to close to the valve seat 12. An intersecting angle .theta. formed by the gas vent passage 10 and the inlet portion of each by-pass passage 15 is an acute angle or a right angle. That is, the angle .theta. between the gas vent passage 10 and each of the by-pass passages 15 at a point where each of the by-pass passages 15 is branched from the gas vent passage 10 is not more than 90.degree.. A mouth portion 16 of the gas vent passage 10 facing the valve chamber 11 is narrowed like a nozzle.
A coil spring 17 is disposed in the gas discharge passage 13 and a hydraulic cylinder 18 for actuating a piston rod 19 connected to the spring 17 is secured to the top of the stationary mold half 3. The valve 14 is urged against the lower end or forward end of the valve chamber 11 by the spring 17. When mold clamping is carried out in the state where the slide valve 14 is located in the valve chamber 11, as illustrated in FIGS. 1 and 2, the valve 14 is pressed downwardly or forwardly by the actions of the cylinder 18 and the coil spring 17 so that it abuts against the forward end of the valve chamber 11, and each of the by-pass passages 15 communicates with the upper or rear portion of the valve chamber 11. In this state, the gas discharge passage 13 communicates with the by-pass passages 15 through the valve chamber 11.
In the above state, when a molten metal or melt is injected into the cavity 7 from the casting hole 8, the gases in the cavity 7 are passed through the gas vent passage 9, the additional gas vent passage 10, the by-pass passages 15, the upper portion of the valve chamber 11 and the gas discharge passage 13, and are discharged from the outlet 20. During the period while a melt 21 is being charged into the cavity 7, as illustrated in FIG. 3A, the slide valve 14 is maintained pressed to the lower portion of the valve chamber 11, and a large quantity of the gases is vented through the by-pass passages 15 as indicated by arrows in FIG. 3A.
When injection of the melt 21 into the cavity 7 is substantially completed, a part of the melt 21 rises in the gas vent passage 10 and impinges against the lower part of forward face of the valve 14, with the result that the valve 14 is pushed up against the downward force of the coil spring 17 by the melt 21, and another part of the melt 21 starts intruding into the by-pass passages 15. The state at this point is illustrated in FIG. 3B.
The slide valve 14 closes the by-pass passages 15 when the melt 21 pushes upward and the flow of the melt 21 is stopped. At this point, the gases which have passed through the by-pass passages 15 are substantially vented and only a slight amount of the gases is left in the vicinity of the valve seat 12. These residual gases have no bad influences on the cast product in cavity 7. The state at this point is illustrated in FIG. 3C.
When the casting or injection operation is completed, the cylinder 18 is operated to lift up the coil spring 17 away from slide valve 14 against the mold, and then the mold opening operation is carried out. The state at this point is illustrated in FIG. 3D. Subsequently, the cast product is removed from the mold by the operation of the push pin 6, and simultaneously, the gas vent passage 10, the lower or forward portion of the valve chamber 11, a solidified metal 21a in the gas vent passage 10, the lower or forward portion of the valve chamber 11, and the by-pass passages 15 as well as the valve 14 are removed together.
The above arrangement utilizes the difference in the specific gravities of the gases and the molten metal (for example, the ratio of the specific gravity of air to molten aluminum is about 1/2000), and, also, the disparity of the force of inertia arising in each material owing to this difference in said specific gravities.
In order to prevent the molten metal 21 rising in the gas vent passage 10 from intruding directly into the by-pass passages 15, and also, to prevent the melt 21 from passing between the valve 14 and the valve seat 12 before the valve 14 is moved rearwardly, the angle .theta. formed by the gas vent passage 10 and the inlet portion of each of the by-pass passages 15 is adjusted to be an acute angle or a right angle. Preferably, the angle .theta. is an acute angle.
At the start of each casting, the slide valve 14 is set in a split half of the valve chamber 11 in the stationary mold half 3, and after the slide valve 14 is pressed downwardly in the lower portion of the valve chamber 11 by the spring 17, the mold is closed. When the slide valve 14 is formed of a material different from the molten metal 21, after withdrawal of the cast product, the slide valve 14 is separated from the cast product and the portion of the solidified metal 21a present in the vicinity thereof, after which it may be reused. When the valve 14 is formed of the same material as the molten metal 21, the used valve 14 is either thrown away or it may be fused together with portion of the solidified metal 21a present in the vicinity of the cast product, such as a sprue and flashes or fins in order to produce a molten metal for casting. When the die casting operation is carried out by using the gas-venting arrangement, a slide valve 14 of the same material as the molten metal 21 can be prepared by said die casting operation using a part of the mold of the die casting machine.
According to the above disclosed art, the following advantages can be obtained.
1. Since the gas discharge passage is shut by the valve which is directly pressed by a molten metal injected into the mold, said metal having advanced directly into the gas vent passage, the valve is thereby moved in the same direction as the advancing direction of the molten metal, the closing of the valve chamber is performed quickly, thus gas venting and prevention of the molten metal from intruding into the valve chamber can be accomplished.
2. Since the gases are sufficiently vented at the injection step, the amount of the gases left in an injection molded product can be drastically reduced, and the running characteristic of the melt, and the pressure resistance and air tightness of the injection-molded product can be remarkably improved.
3. Since formation of fins is reduced in the air vent portion around the cavity, removal of fins need not be carried out and the mold is not damaged, with result that automation of the molding operation can be facilitated and the life of the mold can be prolonged.
4. Since gas venting is sufficiently accomplished, an injection-molded product of a good quality can be obtained under a low injection pressure. Of course, by virtue of this feature, automation of the operation can be facilitated and the life of the mold can be prolonged.
5. Since gas venting is sufficiently accomplished, the allowable ranges of injection conditions can be broadened, and the effects of shortening the time of a trial injection and stabilizing the quality in injection-molded products can be attained. According to the conventional technique, the injection pressure, injection speed and high speed injection-starting position suitable for the gas-venting operation must be determined prior to each series of casting operations. However, a long time is required for determining these variables, which are then gradually changed during the operation. In contrast, according to the disclosed art, since gas venting is sufficiently accomplished, the allowable ranges of injection conditions can be broadened remarkably.
6. There has previously been proposed a method in which air is vented from the cavity through a shallow groove formed on the parting face of the mold half by means of a vacuum device. In this method, however, if the amount of air vented from the cavity is small, air is in turn, introduced from the outside of the mold through gaps in the parting faces of the mold, and a vacuum condition is not produced in the cavity. In contrast, according to the disclosed art, since a large quantity of air is vented, the precision of mating or fitting the parting face of the movable mold half with that of the stationary mold half is not a severe problem. Therefore, if a pressure reduction method is adopted in performing the disclosed art, the effect can be further enhanced.
7. If a nonporous die casting method, where injection is conducted in the cavity with an atmosphere of an active gas, such as oxygen, is adopted in using the disclosed, products of a very high quality can be obtained. In this case, prior to injection of the molten metal, an active gas is introduced into the cavity, from the gas discharge outlet of the gas-venting arrangement, and then injection is performed. Alternatively, active gas can be introduced into the cavity also during injection.
8. Remarkable advantages can be obtained when the disclosed art is applied to die casting of magnesium. In die casting of aluminum there can be adopted a method in which injection is slowly performed to vent the gas from the cavity to the vent portion. However, in the casting of a magnesium alloy, since the solidification speed of the magnesium alloy is very high, low-speed injection is not possible. Instead, soon after the start of the injection operation, the injection speed should be increased to a high level. In the injection operation, a large quantity of the gas contained in the cavity and injection sleeve, which has a volume about 2 times the volume of the cavity, should be vented to the outside of the mold. In die casting of magnesium, since the injection speed should be maintained at a level higher than in die casting of aluminum, inclusion of a relatively large quantity of the gas in an injection-molded product could not be avoided under the prior art. However, when the disclosed art is adopted, since gas venting is sufficiently performed, even in the case of die casting of magnesium, an injection-molded product free of voids can be obtained easily and assuredly.
9. The disclosed art can also be applied to hot chamber-type die casting.
10. According to the conventional technique, after the mold is opened, cooling water or a water-soluble parting agent is sprayed onto the surface of the cavity. When drops of water are left in the mold at the time of mold clamping steam cannot escape, and if an injection is performed in this state, the surface of an injection-molded product is blackened or running of the melt becomes poor, with the result that it becomes impossible to obtain an injection-molded product of high quality. Therefore, mold clamping should be performed after drops of water on the surface of the cavity have been evaporated off by sufficient drying. However, according to the disclosed art, if hot air is fed into the mold through the gas discharge outlet of the gas venting arrangment at the time of mold clamping, steam in the mold is allowed to escape through the injection sleeve. That is, the steam is forced out of the mold by the hot air introduced from the opening end of the gas discharge passage. This feeding of hot air can be conducted not only at the time of mold clamping, but also at the time of the supply of a melt. Accordingly, if an arrangement is made so that hot air is fed into the cavity through the gas-venting arrangement, mold clamping can be accomplished immediately after spraying of the parting agent, and therefore, the operation cycle can be shortened.
11. The gas-venting arrangement can also be used as a permanent means.
The disclosed art provides significant advantages. However, the present invention has discovered that, when the melt part, which is to impinge against the valve 14, flows discontinuously through the gas vent passages 9 and 10, the closing of the valve chamber 11 does not always perform completely and assuredly. This happens when a leading portion of the discontinuous melt part impinges initially against the valve 14, the valve may be forced to move upwardly against the downward force of the coil spring 17 from an open position to a second position, closing the valve chamber 11. Valve 14 may be forced by the downward force of the coil spring 17 to return to the first position to open the valve chamber 11 during a period of time between initial impingement of the leading portion of the melt part until the following portion of the melt part reaches the leading portion at the valve 14.
Under these circumstances, the gas-venting arrangement may encounter the following serious problems. The subsequent or following portion of the melt part approaches valve 14, while the leading portion is in the process of solidification at the front face of the valve 14 and adhering to the valve face as well as to the inner walls of the mouth portion 16 in the vicinity of the valve 14. This will cause the impinging force of the following melt portion to be considerably reduced. This is because the following melt portion impinges pinges against the valve 14, via the leading melt portion which has adhered to the walls and to the valve. Thus, the following melt does not impinge directly against the valve but the leading melt portion. This means that the following melt portion is subjected to a resistance or friction of the leading melt portion generated in the impinging process. This will prevent valve 14 from returning smoothly to the second position, and will result in the valve chamber 11 being not closed completely or the valve 14 not arriving completely at the second position at the final stage of the discontinuous impingement. Further, this will cause axial oscillation of the valve to occur in the process of the discontinuous impingement.
The above mentioned phenomenon will cause the melt to have the opportunity to intrude into the valve chamber 11 through the by-pass passages 15 and through the space gap produced due to the incompleteness of the closing between the valve seat 12 and the valve 14. In such a case, the arrangement neither functions as expected nor attains the expected advantages. Further, there may arise problems that are troublesome, namely, to remove the melt solidified at the valve and at the valve chamber. Thus continued discontinuous impingments will cause the mold machine to be prevented from repeating the injection molding operation smoothly.